In the prior art, a wide range of treatment methods having the purpose of refraction correction in the human eye are known. In this context, the purpose of the operation methods is to alter the cornea selectively so as to influence the light refraction in the eye. A plurality of operation methods are used for this purpose. At present, what is known as laser-assisted in situ keratomileusis, also known as LASIK for short, is the most widespread. In this context, a cornea lamella is initially detached from the cornea surface on one side and folded to the side. This lamella can be detached by means of a mechanical microkeratome, or also by means of what is known as a laser keratome, such as is marketed for example by Intralase Corp., Irvine, USA. After the lamella has been detached and folded to the side, the LASIK operation provides the use of an excimer laser, which removes, by ablation, the corneal tissue which is exposed under the lamella in this manner. After the volume present under the cornea surface has been vaporised in this manner, the cornea lamella is folded back onto the original spot again.
The use of a laser keratome to expose the lamella is advantageous compared to a mechanical blade, since the geometric precision is improved and the frequency of clinically significant complications is reduced. In particular, the lamella can be produced with a much more constant thickness if laser radiation is used. The cut edge is also precisely formed, and this reduces the risk of healing difficulties as a result of this boundary plane which remains even after the operation. However, a drawback of this method is that two different treatment devices have to be used, specifically on the one hand the laser keratome for exposing the lamella and on the other hand the laser which vaporises the corneal tissue.
These drawbacks are eliminated in a method which was implemented very recently by Carl Zeiss Meditec and is known by the abbreviation FLEx. In this lenticule extraction method, a cutting geometry which separates a cornea volume (known as a lenticule) in the cornea is formed in the cornea of the eye by means of a short-pulse laser, preferably a femtosecond laser. This is then removed manually by the operator once the lamella covering the lenticule has been folded to the side. One advantage of this method is that the cutting quality is further improved by the use of the femtosecond laser.
Moreover, only one treatment device is now necessary; the excimer laser is no longer used.
A development of the FLEx method is referred to in the literature as the SMILE method, in which instead of producing a flap, merely a small opening cut provides access to the lenticule positioned under what is known as the cap. The separated lenticule is removed through this small opening cut, damaging the biomechanical integrity of the anterior cornea less than in LASIK, FLEx or PRK. In addition, fewer nerve fibres in the cornea are cut up in this manner, and this has a demonstrably favourable effect on the restoration of the original sensitivity of the cornea surface. The symptom of dry eyes, which often has to be treated after LASIK, is thus reduced in intensity and duration. Other complications following LASIK, which generally relate to the flap (for example folding, epithelial ingrowth in the flap bed), occur more rarely with no flap.
When producing cutting planes in the cornea by means of laser radiation, the optical radiation effect is usually exploited in that an optical aperture is produced by means of individual optical pulses, the duration of which may be between 100 fs and 100 ns. It is also known to introduce individual pulses, the energy of which is below a threshold for an optical aperture, into the tissue or material with an overlap, in such a way that material or tissue separation is achieved in this way too. This idea for producing a cut in the corneal tissue makes a large number of cuts possible.
In the cutting geometry of the SMILE method according to the prior art, it became apparent that, owing to the small opening cut, the two cuts which define the lenticule (cap cut and lenticule cut) cannot be clearly identified in each case, and problems can arise when removing the lenticule. This is carried out using a spatula-shaped instrument (also referred to as a flap lifter) and it may occur that the doctor misses the correct cutting plane and therefore does not correctly separate the lenticule.